Mozart’s musical aesthetics are revealed in a letter to his father about Osmin’s first aria in The Abduction of the Seraglio (Die Entführung aus dem Serail).  The Eighteenth century classical ideals of balance and refinement are evident:

I have sent you only the beginning and the end of the aria.  I think it will prove to be very effective – for Osmin’s anger will be rendered comical by the use of Turkish Music. – In composing the aria I made Fischer’s beautiful deep tones really glisten, in spite of that Salzburger Midas, (1) – The passage Therefore, by the beard of the Prophet (2), etc., is, to be sure, in the same tempo, but with quick notes – and as his anger increases more and more, the allegro assai – which comes just when one thinks the aria is over – will produce an excellent Effect because it is in a different tempo and in a different key.  A person who gets into such a violent rage transgresses every order, moderation, and limit; he no longer knows himself. – In the same way the Music must not know itself – but because passions, violent or not, must never be expressed to the point of disgust, and Music must never offend the ear, even in the most horrendous situations, but always be pleasing, in other words always remain Music, I have not chosen a tone foreign to f, the key of the aria, but one that is friendly to it, not however its nearest relative D minor, but the more remote A minor.

(1) Midas is a mythological king, who had the power of turning everything he touched into gold. Mozart is making a sarcastic remark about the archbishop of Salzburg,
(2) “Drum deim Barte des Propheten.”

– Mozart, in a letter to his father, 26 September 1781.

Cited in: Spaethling, Robert (2000) Mozart’s Letters; Mozart’s Life.  London: Faber and Faber, p.286.